The present invention relates to a variable magnification optical system and, more particularly, to an afocal variable magnification optical system.
In conventional variable magnification optical systems, in order to keep a focusing surface at a constant position during a magnification varying operation, at least two lens groups must be moved along an optical axis. For this reason, the two lens groups must be spaced apart exceeding a constant distance so as not to collide with each other. Due to this requirement, the lens system cannot be rendered compact. In addition, since at least one lens group must be nonlinearly moved, an expensive moving mechanism is required.
In particular, in order to constitute an afocal variable magnification optical system, at least one lens group is required, in addition to the two moving lens groups. When the afocal optical system is constituted by two lens groups, an image space focal point of a lens group at the side of an image and an object focal point of a lens group at the side of an image must coincide with each other, and magnification cannot be varied by movement of the lens groups. For this reason, in order to realize a conventional afocal variable magnification optical system which keeps the focusing surface at a constant position, for example, as described in Japanese Patent Laid-Open No. 149915/1980, a first moving lens group for varying magnification, a second moving lens group for compensating for movement of an image position associated with a variation in magnification and a third lens group for converting the overall system into an afocal system are needed. Therefore, as compared to a focal variable magnification optical system, an afocal variable magnification optical system becomes expensive, and an extra space is required, thus preventing a compact system.